Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest.
Where replacement of a heart valve is indicated, the dysfunctional valve is typically cut out and replaced with either a mechanical valve, or a tissue or bioprosthetic-type valve. Bioprosthetic-type valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. The most common bioprosthetic-type valves are constructed with whole porcine (pig) valves, or with separate leaflets cut from bovine (cow) pericardium.
Although so-called stentless valves, comprising a section of xenograft (e.g., porcine) aorta and valve, are available, the most widely used valves include some form of artificial leaflet support. One such support is an elastic support frame, sometimes called a wireform or stent, which has a plurality (typically three) of large radius U-shaped cusps supporting the cusp region of the leaflets of the bioprosthetic tissue (i.e., either a whole valve or three separate leaflets). The free ends of each two adjacent cusps converge somewhat asymptotically to form upstanding commissures that terminate in U-shaped tips, each being curved in the opposite direction as the cusps, and having a relatively smaller radius. The support frame typically describes a conical tube with the commissure tips at the small diameter end. This provides an undulating reference shape to which a fixed edge of each leaflet attaches (via components such as fabric and sutures) much like the natural fibrous skeleton in the aortic annulus. Therefore, the alternating cusps and commissures mimic the natural contour of leaflet attachment. Importantly, the wireform provides continuous support for each leaflet along the cusp region so as to better simulate the natural support structure.
The support frame is typically a non-ferromagnetic metal such as ELGILOY (a Co—Cr alloy) that possesses substantial elasticity. A common method of forming metallic support frames is to bend a wire into a flat (two-dimensional) undulating pattern of the alternating cusps and commissures, and then roll the flat pattern into a tube using a cylindrical roller. The free ends of the resulting three-dimensional shape, typically in the asymptotic region of the cusps, are then fastened together using a tubular splice that is plastically crimped around the ends. See FIGS. 3 and 4 of U.S. Pat. No. 6,296,662 for a support frame that is crimped together at a cusp midpoint.
Some valves include polymeric support frames rather than metallic, for various reasons. For example, U.S. Pat. No. 5,895,420 discloses a plastic support frame that degrades in the body over time. Despite some favorable attributes of polymeric support frames, for example the ability to mold the complex support frame shape, conventional metallic support frames are generally preferred for their elastic properties, and have a proven track record in highly successfully heart valves. For example, the CARPENTIER-EDWARDS Porcine Heart Valve and PERIMOUNT Pericardial Heart Valve available from Edwards Lifesciences LLC both have ELGILOY support frames and have together enjoyed the leading worldwide market position since 1976.
A conventional heart valve replacement surgery involves accessing the heart in the patient's thoracic cavity through a longitudinal incision in the chest. For example, a median sternotomy requires cutting through the sternum and forcing the two opposing halves of the rib cage to be spread apart, allowing access to the thoracic cavity and heart within. The patient is then placed on cardiopulmonary bypass which involves stopping the heart to permit access to the internal chambers. Such open heart surgery is particularly invasive and involves a lengthy and difficult recovery period.
Some attempts have been made to enable less traumatic delivery and implantation of prosthetic heart valves. For instance, U.S. Pat. No. 4,056,854 to Boretos discloses a radially collapsible heart valve secured to a circular spring stent that can be compressed for delivery and expanded for securing in a valve position. Also, U.S. Pat. No. 4,994,077 to Dobbin describes a disk-shaped heart valve that is connected to a radially collapsible stent for minimally invasive implantation.
Recently, a great amount of research has been done to reduce the trauma and risk associated with conventional open heart valve replacement surgery. In particular, the field of minimally invasive surgery (MIS) has exploded since the early to mid-1990s, with devices now being available to enable valve replacements without opening the chest cavity. MIS heart valve replacement surgery still typically requires bypass, but the excision of the native valve and implantation of the prosthetic valve are accomplished via elongated tubes or cannulas, with the help of endoscopes and other such visualization techniques.
Some examples of more recent MIS heart valves are shown in U.S. Pat. No. 5,411,552 to Anderson, et al., U.S. Pat. No. 5,980,570 to Simpson, U.S. Pat. No. 5,984,959 to Robertson, et al., U.S. Pat. No. 6,425,916 to Garrison, et al., and PCT Publication No. WO 99/334142 to Vesely.
Although these and other such devices provide various ways for collapsing, delivering, and then expanding a heart valve per se, none of them disclose much structural detail of the valve itself. For instance, the publication to Vesely shows a tissue leaflet structure of the prior art in FIG. 1, and an expandable inner frame of the invention having stent posts in FIGS. 3A-3C. The leaflets are mounted to the stent posts 22 in a manner similar to that shown in FIG. 1. Likewise, Anderson describes mounting a porcine valve inside of an expandable stent by means of a suitable number of sutures to form the cardiac valve prosthesis 9 shown in FIG. 2. Such general disclosures stop short of explaining how to construct a valve in a manner that maximizes long-term efficacy. In particular, the particular means of attaching the leaflets to the MIS stent is critical to ensure the integrity and durability of the valve once implanted. All of the prior art MIS valves are inadequate in this regard. Furthermore, use of conventional support stents or wireforms is difficult in MIS valves because of the need to compress the valve into a relatively small diameter delivery package, which creates material challenges.
Some MIS valves of the prior art are intended to be used without removing the natural valve leaflets. Sometimes the natural leaflets are heavily calcified, and their removal entails some risk of plaque particles being released into the bloodstream. Therefore, some of the MIS valves are designed to expand outward within the annulus and native leaflets, and compress the leaflets against the annulus. The relatively uneven surface of the calcified annulus and leaflets creates sizing problems and may complicate the delivery and placement steps. Prior art MIS valves are essentially tubular stents embellished with a native xenograft valve. The implant methodology is simply the conventional balloon expansion technique or pushing a self-expanding version from the end of a catheter. Minimal control over the placement of the valve is provided or contemplated.
Despite some advances in MIS valve design, there remains a need for an MIS valve that is durable and which has a more flexible delivery and implantation methodology.